Field of the Invention
The present invention relates to a driver for an electrochromic element, a method for driving an electrochromic element, an optical filter, an imaging device, a lens unit, and a window component. In particular, the present invention relates to a driver for an organic electrochromic element configured to adjust the tone of the element, a method for driving an organic electrochromic element with such a driver, and an optical filter, an imaging device, and a window component in which such a driver is used.
Description of the Related Art
Electrochromism (EC) is a phenomenon in which a reversible electrochemical reaction (oxidation or reduction) induced upon application of voltage changes the optical absorption range of a substance and thereby makes the substance colored or colorless. An electrochemically colored/erased element that works on electrochromism is referred to as an electrochromic (EC) element and is expected to be used as a light-controlling element with varying light transmittance.
Known EC elements include ones in which a metal oxide, such as WO3, is used as EC material, EC elements in which a conductive polymer is used, and EC elements in which an organic small molecule, such as viologen, is used. In particular, an organic EC element in which a low-molecular-weight organic material turns colored/colorless in the form of solution is known to have advantages such as a sufficiently high contrast ratio in the colored state and high transmittance in the colorless state. This type of EC element is also known to be advantageous in that it can have any desired color tone by containing multiple materials with different absorption wavelengths.
The use of an EC element in an optical filter requires tone control drive that allows for the control of the amount of light that passes through the filter. As a driving method for tone control, Japanese Patent Laid-Open No. 11-109423 discloses a PWM driving method that includes applying a voltage pulse. In this driving method the tone is controlled through the control of the durations per pulse for which the oxidation and the reduction of the organic EC material proceed.
Japanese Patent Laid-Open No. 11-316396 discloses a method for preventing EC material in an EC element from remaining colored at the start of service of the EC element, and this method includes resetting the EC element to the initial state at the start or end of service of the element.
A non-patent document (Michael G. Hill, Jean-Francois Penneau, Baruch Zinger, Kent R. Mann, and Larry L. Miller, “Oligothiophene Cation Radicals. π-Dimers as Alternatives to Bipolarons in Oxidized Polythiophenes,” Chemistry of Materials, 1992, 4, 1106-1113) reports a material whose radical species forms an assembly while the material is colored through oxidation.
The control of the tone of an organic EC element through quantitative regulation of electrochemical reaction has been found disadvantageous because of the following problems that occur with a single material or between multiple materials.
As disclosed in the aforementioned non-patent document, some materials have a radical species that forms an assembly (a dimer) while the material is colored through reaction. The electronic state of such an assembly is different from that of the radical species of the material, and thus the radical form and the assembly exhibit different absorption profiles. Research by the inventors has found that the absorption spectrum of such a material varies because the behavior of absorption changes of the radical species and the assembly during coloring is different from that during erasing. It is therefore difficult to control the tone while maintaining the absorption spectrum in both directions, i.e., the coloring direction and the erasing direction, when using a material that forms an assembly.
In cases where multiple materials are mixed that turn into cation through oxidation from the neutral state and return to the neutral state through back reduction, the absorption ratios between the materials during coloring are different from those during back reduction because the materials have different oxidation voltages and their oxidized forms have different back reduction voltages. In general, oxidation of a material is more likely to occur with increasing positive difference between the oxidation voltage of the material and the voltage of the electrode that acts on the material, and back reduction is more likely to occur with increasing negative difference between the back reduction voltage of the material and the voltage of the electrode that acts on the material. Trying to oxidize multiple materials together therefore results in the material that has the lowest oxidation voltage going into reaction faster than all other materials, which have higher oxidation voltages. When back-reducing the cations as oxidized forms of multiple materials together, however, the reaction is not advantageous to the same material because the back reduction voltage of this material is lower than that of the other materials.